1. Field of the Invention
The invention relates generally to downhole recorders for use with a downhole assembly.
2. Background Art
A downhole assembly is used within a borehole, for example for drilling the borehole itself, or for characterizing a formation surrounding the borehole. The downhole assembly, when lowered into the borehole, may be subject to extreme conditions, e.g. high pressures, high temperatures. The downhole assembly may subsequently be worn down or damaged. Maintenance operations are hence performed on the downhole assembly.
In order to evaluate a state of the downhole assembly, operating data, e.g. a date of repair, a nature of the repair, a peak of temperature measured downhole, are traditionally written on paper or into a computer file. The writing of the operating data allows to constitute a database of the operating data. A lifetime of the downhole assembly or of a sub-assembly of the downhole assembly may be evaluated from the operating database.
Sensors located downhole allow to perform measurements of environment parameters, e.g. temperature, shock events, vibration events, humidity rate, number of ON/OFF cycles, pressure, supply voltage and currents, flow rate of a liquid, rotating velocity of a collar of the downhole assembly. The sensors are traditionally read at surface following a raising operation.
A recorder mounted on the downhole assembly allows to provide a downhole storage of the operating data. Typically, the sensors are connected to the recorder and measured environment data are written into the recorder.
When the downhole assembly is raised up to a surface, the recorder is read and at least a portion of a content of the recorder is transferred into a computer.
FIG. 1 illustrates an example of a downhole assembly from prior art. The downhole assembly 11 is lowered into a borehole 12. In the example represented in FIG. 1, the downhole assembly 11 is lowered by means of a wireline cable 17. The downhole assembly 11 comprises a logging tool (not represented) allowing to characterize a formation 18 surrounding the borehole 12.
Sensors 13 allow to measure environment data that are subsequently written into a recorder 15. The measured environment data may be read at the recorder when the downhole assembly is raised to the surface. Alternatively, as represented in FIG. 1, an electrical cable 19 allows to read the measured environment data from a computer 16 located at the surface.
An operating database may be constructed from the read environment data and from maintenance data stored either in the recorder 15 or directly in the computer 16. A lifetime of the downhole assembly 11 may be evaluated from the operating database, according to a reliability model.
The downhole assembly 11 usually comprises at least one sub-assembly 14, e.g. a collar (not represented), a drill bit (not represented). The sensors 13 themselves may also be considered as a sub-assembly. The computer 16 may allow to evaluate a lifetime of each one of the sub-assemblies 14. A plurality of sub-assembly reliability models, each sub-assembly reliability model allowing to evaluate the lifetime of a determined sub-assembly, may be used for that purpose.